From the tomcat 6 documentation it looks like you can set the classname of things like the Context (http://tomcat.apache.org/tomcat-6.0-doc/config/context.html), Engine (http://tomcat.apache.org/tomcat-6.0-doc/config/engine.html), and Host (http://tomcat.apache.org/tomcat-6.0-doc/config/host.html), but not the Connector.
Am I missing something here or is it just not possible?